Oil-bearing potential and hydrocarbon occurrence characteristics of shale in Paleogene Hetaoyuan Formation in Biyang Sag, Nanxiang Basin

Abstract

High-quality lacustrine shale beds within the Paleogene Hetaoyuan Formation in the Biyang Sag, Nanxiang Basin, exhibit significant potential for shale oil resources. Previous studies on the oil-bearing potential and hydrocarbon occurrence of shales in the Biyang Sag were relatively limited, potentially hindering advancements for shale oil exploration. This study focuses on the shales found in the third sub-member of the third member within the Hetaoyuan Formation in well Y1, which locates in the southern area of the Biyang Sag. Through various geochemical analyses such as Rock-Eval pyrolysis, multi-temperature pyrolysis and X-ray diffraction, this research systematically investigates the shale oil contents, hydrocarbon occurrence characteristics, and influencing factors. These results indicate that the lithofacies assemblages of the shale primarily consist of felsic shale facies, dolomitic shale facies and mixed shale facies, with a well-developed laminar structure. The source rock quality is classified as good to excellent, and the thermal maturity stage of oil window. The predominant organic maceral is sapropelic, with organic matter types ranging from Ⅰ to Ⅱ1. Oil content increases with burial depth, ranged from adsorbed hydrocarbons in the upper section to free hydrocarbons in the lower section. Detrital mineral content and total organic carbon (TOC) content are identified as the main factors influencing free and adsorbed hydrocarbon content, respectively. Overall, the oil saturation index (OSI) of the shale in the lower section of the Hetaoyuan Formation exceeds 100 mg/g, with an average free hydrocarbon content over 3 mg/g, indicating promising prospects for shale oil exploration and development.

Keywords

• shale oil
• oil-bearing potential
• hydrocarbon occurrence state
• hetaoyuan formation
• paleogene
• biyang sag
• nanxiang basin